欧罗巴星陨石坑对冰层厚度的制约

欧罗巴（木卫二）是目前最有可能存在地外生物的星体，是空间探测计划的重点对象.欧罗巴冰层厚度直接制约着生物存在的可能性.另一方面，冰层厚度控制着温度结构、流变特性，进而制约着冰体构造特征及演化过程.前人基于板的挠曲，陨石坑分析，热-力学分析等方法得到的冰层厚度变化范围很大（小于1 km到大于30 km）.基于前人的研究，我们通过研究欧罗巴陨石坑的松弛过程进而约束冰层的厚度.本文将冰视为非牛顿流体，修正了前人在引用冰体实验数据过程中存在的不足.依据前人的研究思路，基于有限单元法及更新网格技术，取新近形成的陨石坑形状为初始几何模型，针对不同厚度的冰层，对欧罗巴最大陨石坑的松弛过程进行了动力学模拟.模拟结果显示：1）冰层越厚，所需松弛时间越长；2）冰层越厚，陨石坑附近的黏度越高，这是松弛时间相对较长的直接原因.本文认为欧罗巴冰层的厚度大于20 km.值得注意的是，作为端元模型，本文模型中冰层与基岩直接接触，后续研究将进一步考虑其他模型.

Constraint of impact craters on ice thickness on the Europa

Up to now,Europa is the most likely planet with extraterrestrial organism.Therefore,it is the focus of many space exploration projects.Ice thickness of Europa restricts the possibility of the existence of life directly.Meanwhile,ice thickness controls the temperature structure and rheological properties of Europa,which further restricts the tectonic characteristics and evolution processes of the ice-layer.Previous studies (based on plate flexure,crater analysis and thermo-mechanical analysis) yielded highly variable ice thicknesses (<1 km to >30 km) of Europa.Following these previous studies,we attempt to determine the ice thickness of Europa by analyzing the relaxation of Europa ice craters.We apply the FEM (finite element method) and updating grid technology for the numerical simulation.In addition,the ice-layer is treated as a non-Newtonian fluid,which thereby improves the traditional methods used in the previous studies.The shape of a newly formed crater is adopted as the geometry of the initial model,which is further used to simulate the relaxation process of the largest crater of Europa.The model results show that:1) The thicker the ice-layer of Europa,the longer time for the relaxation; 2) the thicker the ice-layer of Europa,the higher viscosity near the craters.Finally,the numerical studies indicate that the ice thickness of Europa should be greater than 20 km.It is worth noting that the ice-layer and the bedrock beneath it are in a direct contact in the current models,so it may need further studies considering variable contact relationships.